Method for channel sounding in wireless local area network and apparatus for the same

ABSTRACT

A channel sounding method in a wireless local area network (WLAN) system is provided. The method, performed by a transmitter, includes transmitting a null data packet announcement (NDPA) frame to a receiver to initiate a channel sounding procedure; transmitting a null data packet (NDP) to the receiver and receiving a feedback frame. The feedback frame includes a plurality of segment frames and a channel feedback report. The channel feedback report is split into a plurality of feedback segments. Each of the plurality of feedback segments is respectively included in each of the plurality of segment frames. The each of the plurality of segment frames includes a first-segment subfield indicating whether the each of the plurality of feedback segment included is a first segment and a remaining-segment subfield indicating the number of remaining feedback segments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending Application No.13/307,774 filed on Nov. 30, 2011, which claims the benefit of priorityof U.S. Provisional application No. 61/418,417 filed on Dec. 1, 2010,U.S. Provisional application No. 61/423,589 filed on Dec. 16, 2010, andKorean Patent Application No. 10-2011-0072845 filed on Jul. 22, 2011.The entire contents of all of the above applications are herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to a wireless local area network (WLAN)system, and more particularly, to a channel sounding procedure betweenstations (STAs) in the WLAN system and an apparatus for supporting theprocedure.

2. Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. A next-generation WLAN system supportinga very high throughput (VHT) is a next version of the IEEE 802.11n WLANsystem, and is one of IEEE 802.11 WLAN systems which have recently beenproposed to support a data processing rate of above 1 Gbps in a MACservice access point (SAP).

The next-generation WLAN system supports the transmission of aMulti-User Multiple Input Multiple Output (MU-MIMO) scheme in which aplurality of non-AP STAs accesses a radio channel at the same time inorder to efficiently use the radio channel. According to the MU-MIMOtransmission scheme, an AP can transmit a frame to one or moreMIMO-paired STAs at the same time.

The AP and the plurality of MU-MIMO paired STAs may have differentcapabilities. In this case, a supportable bandwidth, modulation codingscheme (MCS), forward error correction (FEC), etc., may vary dependingon an STA type, usage, channel environment, etc.

In the WLAN system, an AP and/or an STA can acquire information on achannel to be used to transmit a frame to a reception target AP and/orSTA. This can be performed by using a channel sounding procedure. Thatis, a process in which a transmitter requests a receiver to send channelinformation to be used for frame transmission and reception and thereceiver estimates a channel and feeds back channel information thereofto the transmitter can be performed before transmitting and receiving adata frame. Meanwhile, since the next generation WLAN system employs awider channel bandwidth and a MU-MIMO transmission scheme, a more amountof channel information can be received from a transmission target APand/or STA. In order to transmit a more amount of feedback information,the transmission target AP and/or STA needs to access to a channel for alonger period of time. When interference occurs in a part of thefeedback information transmitted and received during this period, thereis a need to discard whole feedback information and thus new feedbackinformation is necessary. This may cause deterioration in terms ofefficiency of resource usage and reliability of a channel soundingprocedure. Accordingly, there is a need for a channel sounding methodcapable of solving the aforementioned problem in a next generation WLANsystem.

SUMMARY

The present invention provides a sounding method performed by a station(STA) in a next generation wireless local area network (WLAN) systemsupporting a multi user-multiple input multiple output (MU-MIMO)transmission scheme.

In an aspect, a channel sounding method in a wireless local area network(WLAN) system is provided. The method, performed by a transmitter,includes transmitting a null data packet announcement (NDPA) frame to areceiver to initiate a channel sounding procedure; transmitting a nulldata packet (NDP) to the receiver and receiving a feedback frame. Thefeedback frame includes a plurality of segment frames and a channelfeedback report, the channel feedback report is split into a pluralityof feedback segments, each of the plurality of feedback segments isrespectively included in each of the plurality of segment frames. Theeach of the plurality of segment frames includes a first-segmentsubfield indicating whether the each of the plurality of feedbacksegment included is a first segment and a remaining-segment subfieldindicating the number of remaining feedback segments.

The method may further include determining whether at least one feedbacksegment is missed on the basis of the first-segment subfield and theremaining-segment subfield and transmitting retransmission requestinformation for the missed feedback segment, when a missed feedbacksegment is detected.

The plurality of feedback segments may be equal in size except for alast feedback segment.

The feedback frame may be an aggregate medium access control (MAC)protocol data unit (A-MPDU) transported as a data unit managed in aphysical layer, and the segment frame is a MAC protocol data unit (MPDU)which is mutually exchanged in a MAC entity of the WLAN system.

If a size of the channel feedback report is greater than or equal to aspecific value, the channel feedback report may be split into theplurality of feedback segments.

The channel feedback report may include feedback information used by thetransmitter to determine a steering matrix.

The channel feedback report may further include feedback information fora signal to ratio (SNR) for each spatial stream.

The retransmission request information may be a bitmap sequence in whicha bit value corresponding to an index of the missed feedback segment isset to 1.

The method may further include receiving the missed feedback segmentfrom the receiver.

In another aspect, a channel sounding method in a WLAN system isprovided. The method, performed by a receiver, includes receiving anNDPA frame transmitted by a transmitter to initiate a channel soundingprocedure, generating a feedback frame including a channel feedbackreport and transmitting the feedback frame to the transmitter. The NDPAframe includes an identifier of the receiver. The generating of thefeedback frame includes splitting the channel feedback report into aplurality of feedback segments and generating the feedback frame byincluding a plurality of segment frames in the feedback frame. Each ofthe plurality of segment frames respectively includes each of theplurality of feedback segments, a first-segment subfield indicatingwhether the each of the plurality of feedback segments included is afirst segment and a remaining-segment subfield indicating the number ofremaining feedback segment.

In still another aspect, a wireless apparatus is provided. The apparatusincludes a transceiver transmitting and receiving a frame and aprocessor operationally coupled to the transceiver. The processor isconfigured for: transmitting a null data packet announcement (NDPA)frame to a receiver to initiate a channel sounding procedure;transmitting a null data packet (NDP) to the receiver; and receiving afeedback frame. The feedback frame includes a plurality of segmentframes and a channel feedback report. The channel feedback report issplit into a plurality of feedback segments. Each of the plurality offeedback segments is respectively included in each of the plurality ofsegment frames. The each of the plurality of segment frames includes afirst-segment subfield indicating whether the each of the plurality offeedback segment included is a first segment and a remaining-segmentsubfield indicating the number of remaining feedback segments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 shows an example of a PPDU format according to an embodiment ofthe present invention.

FIG. 3 is a diagram showing a channel sounding method using an NDP in anext generation WLAN system.

FIG. 4 shows an example of corruption occurrence in a part of receivingsignal.

FIG. 5 shows an example of a feedback frame applicable to an embodimentof the present invention.

FIG. 6 shows an example of a channel sounding method according to anembodiment of the present invention.

FIG. 7 is a diagram showing a format of a feedback control field thatcan be included in a segment feedback frame according to an embodimentof the present invention.

FIG. 8 is a diagram showing a channel information field format that canbe included in a segment feedback frame according to an embodiment ofthe present invention.

FIG. 9 shows a conceptual location of a subcarrier tone to be fed backaccording to an embodiment of the present invention.

FIG. 10 shows an example of an NDPA frame format according to anembodiment of the present invention.

FIG. 11 is a block diagram showing a wireless apparatus to which anembodiment of the present invention is applicable.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, A WLAN system includes one or more Basic ServiceSet (BSSs). The BSS is a set of stations (STAs) which can communicatewith each other through successful synchronization. The BSS is not aconcept indicating a specific area

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STA5, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a plurality of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to as a mobile terminal, a wireless device, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, or simply a user. For convenience ofexplanation, the non-AP STA will be hereinafter referred to the STA.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between STAs in an infrastructure BSS including the AP isperformed via the AP in principle, the STAs can perform directcommunication when a direct link is set up. The AP may also be referredto as a central controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe interconnected by the use of the DS. An extended service set (ESS) isa plurality of BSSs connected by the use of the DS. APs and/or STAsincluded in the ESS can communicate with each another. In the same ESS,an STA can move from one BSS to another BSS while performing seamlesscommunication.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an AP and/or an STA sensesa wireless channel or medium before starting transmission. As a resultof sensing, if it is determined that the medium is in an idle status,frame transmission starts by using the medium. Otherwise, if it issensed that the medium is in an occupied status, the AP and/or the STAdoes not start its transmission but sets and waits for a delay durationfor medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the AP and/or the STA directlysenses the medium. The virtual carrier sensing is designed to compensatefor a problem that can occur in medium access such as a hidden nodeproblem. For the virtual carrier sending, the MAC of the WLAN systemuses a network allocation vector (NAV). The NAV is a value transmittedby an AP and/or an STA, currently using the medium or having a right touse the medium, to anther AP or another STA to indicate a remaining timebefore the medium returns to an available state. Therefore, a value setto the NAV corresponds to a period reserved for the use of the medium byan AP and/or an STA transmitting a corresponding frame.

The AP and/or the STA may perform a procedure of exchanging a request tosend (RTS) frame and a clear to send (CTS) frame to announce that itintends to access a medium. The RTS frame and the CTS frame includeinformation indicating a time duration reserved for access of a radiomedium required to transmit and receive an acknowledgement (ACK) framewhen an actual data frame transmission and reception ACK is supported.Upon receiving an RTS frame transmitted from an AP and/or an STAintending to transmit a frame or upon receiving a CTS frame transmittedfrom a frame transmission target STA, another STA can be configured notto access to the medium for the time duration indicated by theinformation included in the RTS/CTS frame. This can be implemented byconfiguring an NAV for the time duration.

Unlike the conventional WLAN system, the next generation WLAN systemrequires a higher throughput. This is called a very high throughput(VHT). For this, the next generation WLAN system intends to support 80MHz, contiguous 160 MHz, non-contiguous 160 MHz bandwidth transmissionand/or higher bandwidth transmission. In addition, a multi user-multipleinput multiple output (MU-MIMO) transmission scheme is provided for thehigher throughput. In the next generation WLAN system, an AP cantransmit a data frame simultaneously to at least one or more MIMO-pairedSTAs. In the WLAN system of FIG. 1, an AP 10 can transmit datasimultaneously to an STA group including at least one or more STAs amonga plurality of STAs 21, 22, 23, 24, and 30 associated with the AP 10. Inthis case, data transmitted to each STA can be transmitted throughdifferent spatial streams. A data frame transmitted by the AP 10 can bereferred to as a physical layer convergence procedure (PLCP) protocoldata unit (PPDU) generated and transmitted in a physical layer (PHY) ofthe WLAN system. It is assumed in the embodiment of the presentinvention that a transmission target STA group which is MU-MIMO pairedwith the AP 10 is the STA1 21, the STA2 22, the STA3 23, and the STA424. In this case, the spatial stream may not be allocated to a specificSTA in the transmission target STA group and thus data may not betransmitted. Meanwhile, it is assumed that the STAa 30 is an STA whichis associated with the AP but is not included in the transmission targetSTA group.

FIG. 2 shows an example of a PPDU format according to an embodiment ofthe present invention.

Referring to FIG. 2, a PPDU 200 includes an L-STF field 210, an L-LTFfield 220, an L-SIG field 230, a VHT-SIGA field 240, a VHT-STF field250, a VHT-LTF field 260, a VHT-SIGB field 270, and a data field 280.

A PLCP sub-layer constituting a PHY converts a PLCP service data unit(PSDU) delivered from a MAC layer into the data field 280 by appendingnecessary information to the PSDU, generates the PPDU 200 by appendingseveral fields such as the L-STF field 210, the L-LTF field 220, theL-SIG field 230, the VHT-SIGA field 240, the VHT-STF field 250, theVHT-LTF field 260, the VHT-SIGB field 270, or the like, and delivers thePPDU 200 to one or more STAs through a physical medium dependent (PMD)sub-layer constituting the PHY.

The L-STF field 210 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF field 220 is used for channel estimation for demodulation ofthe L-SIG field 230 and the VHT-SIGA field 240.

The L-SIG field 230 is used when an L-STA receives the PPDU to acquiredata.

The VHT-SIGA field 240 includes control information for interpreting thereceived PPDU 200 as common control information required for VHT-STAswhich are MIMO-paired with an AP. The VHT-SIGA field 240 includesinformation on a spatial stream for each of the plurality of MIMO-pairedSTAs, bandwidth information, identification information regardingwhether space time block coding (STBC) is used, a group identifier for atransmission target STA group, information regarding a spatial streamallocated to an STA included in a transmission target group STAindicated by the group identifier, information regarding a short guardinterval (GI) of the transmission target STA, coding information,modulation and coding scheme (MCS) information, information indicatingwhether beamforming is performed, and CRC related information. Herein,the group identifier may include whether a currently used MIMOtransmission method is MU-MIMO or SU-MIMO. The VHT-SIGA field 240 may betransmitted by using two OFDM symbols. In this case, a field related toa symbol that comes first can be referred to as a VHT-SIGA1, and a fieldrelated to a subsequent symbol can be referred to as a VHT-SIGA2 field.

The VHT-STF field 250 is used to improve performance of AGC estimationin MIMO transmission.

The VHT-LTF field 260 is used when the STA estimates a MIMO channel.Since the next generation WLAN system supports MU-MIMO, the VHT-LTFfield 260 can be configured by the number of spatial streams on whichthe PPDU 200 is transmitted. In addition, when full channel sounding issupported and is performed, the number of VHT-LTFs may increase.

The VHT-SIGB field 270 includes dedicated control information requiredwhen the plurality of MIMO-paired STAs receive the PPDU 200 to acquiredata. Therefore, the STA may be designed such that the VHT-SIGB field270 is decoded only when the common control information included in theVHT-SIGB field 270 indicates that the currently received PPDU 200 istransmitted using MU-MIMO transmission. On the contrary, the STA may bedesigned such that the VHT-SIGB field 270 is not decoded when the commoncontrol information indicates that the currently received PPDU 200 isfor a single STA (including SU-MIMO).

The VHT-SIGB field 270 includes length information of a PSDU included inthe data field transmitted to each STA, MSC information, and tailrelated information included in the data field. Further, the VHT-SIGBfield 270 includes information on encoding and rate-matching. A size ofthe VHT-SIGB field 270 may differ according to the MIMO transmissionmethod (MU-MIMO or SU-MIMO) and a channel bandwidth used for PPDUtransmission.

The data field 280 includes data intended to be transmitted to the STA.The data field 280 includes a service field for initializing a scramblerand a PLCP service data unit (PSDU) to which a MAC protocol data unit(MPDU) of a MAC layer is delivered, a tail field including a bitsequence required to reset a convolution encoder to a zero state, andpadding bits for normalizing a length of the data field.

In the WLAN system of FIG. 1, if the AP 10 intends to transmit data tothe STA1 21, the STA2 22, and the STA3 23, then a PPDU may betransmitted to an STA group including the STA1 21, the STA2 22, the STA323, and the STA4 24. In this case, as shown in FIG. 2, no spatial streammay be allocated to the STA4 24, and a specific number of spatialstreams may be allocated to each of the STA1 21, the STA2 22, and theSTA3 23 and then data can be transmitted according thereto. In theexample of FIG. 2, one spatial stream is allocated to the STA1 21, threespatial streams are allocated to the STA2 22, and two spatial streamsare allocated to the STA3 23.

One of the most significant features of the next generation WLAN systemis that an overall system throughput can be improved according to aMU-MIMO transmission scheme in which several spatial streams aretransmitted to a plurality of STAs by using multiple antennas. In asituation where the plurality of STAs are present, an AP intending toperform data transmission transmits a PPDU through a beamformingprocedure to transmit data to a transmission target STA group.Therefore, since an AP and/or an STA intending to transmit the PPDUrequires channel information for each transmission target STA, a channelsounding procedure is necessary to acquire the channel information.

Two schemes are supported for the channel sounding procedure. One is ascheme based on a normal PPDU including a data field, and the other is ascheme based on a null data packet (NDP) not including the data field.Hereinafter, the NDP may also be referred to as a sounding frame.

FIG. 3 is a diagram showing a channel sounding method using an NDP in anext generation WLAN system. In this embodiment, an AP performs channelsounding on three transmission target STAs in order to transmit data tothe three transmission target STAs. However, the AP may perform channelsounding on one STA.

Referring to FIG. 3, an AP 310 transmits an NDP announcement (NDPA)frame to an STA1 321, an STA2 322, and an STA3 323 (step S310). The NDPAframe is for announcing information for identifying an STA fortransmitting a feedback frame in response to an NDP transmittedsubsequently. The AP 310 transmits the NDPA frame by inserting an STAinformation field including information on a sounding target STA intothe NDPA frame. One STA information field may be included for eachsounding target STA. The NDPA frame can be referred to as a soundingannouncement frame.

As shown in FIG. 3, the AP 310 broadcasts the NDPA frame if the NDPAframe is transmitted to at least one sounding target STA for MU-MIMOchannel sounding. On the other hand, if the NDPA frame is intended to betransmitted to one sounding target STA for SU-MIMO channel sounding,recipient address information included in the NDPA frame may betransmitted in a unicast manner by configuring it as a MAC address ofthe sounding target STA.

Table 1 below shows an example of an STA information field formatincluded in the NDPA frame.

TABLE 1 Subfield Description AID Contain AID of sounding target STAFeedback Type Indicate feedback request type for sounding target STA Setto 0 for SU-MIMO Set to 1 for MU-MIMO Nc Index Indicate requestedfeedback dimension In case of MU-MIMO: Set to 0 if Nc = 1 Set to 1 if Nc= 2 Set to 2 if Nc = 3 Set to 3 if Nc = 4 Set to 4 if Nc = 5 Set to 5 ifNc = 6 Set to 6 if Nc = 7 Set to 7 if Nc = 8 In case of SU-MIMO, used asa reserved subfield (set to 1)

In Table 1 above, Nc denotes the number of columns of beamformingfeedback matrices among a plurality of pieces of feedback informationtransmitted by the sounding target STA to the AP in response to areceived NDP.

Upon receiving the NDPA frame, STAs may confirm an AID subfield valueincluded in the STA information field, and may determine whether theSTAs are sounding target STAs. In the embodiment of FIG. 3, the NDPAframe may include an STA information field including an AID of the STA1321, an STA information field including an AID of the STA2 322, and anSTA information field including an AID of the STA3 323.

Subsequent to NDPA frame transmission, the AP 310 transmits an NDP tothe sounding target STA (step S320). The NDP may have a format obtainedby excluding a data field from the PPDU format of FIG. 2. The NDP frameis subjected to beamforming by the AP 310, and is transmitted to thesounding target STA by using at least one or more spatial streams.Therefore, sounding target STAs 321, 322, and 323 may estimate channelson the basis of a VHT-LTF of the NDP.

As control information included in the NDP in NDP transmission, lengthinformation indicating a length of a PSDU included in the data field ora length of an aggregate-MAC protocol data unit (A-MPDU) included in thePSDU is set to 0, and information indicating the number of transmissiontarget STAs is set to 1. A group ID indicating whether a transmissionscheme used for NDP transmission is MU-MIMO or SU-MIMO and indicating atransmission target STA group is set to a value indicating SU-MIMOtransmission. Information indicating the number of spatial streamsallocated to the transmission target STA is set to a value correspondingto the number of spatial streams transmitted to the transmission targetSTA through MU-MIMO or SU-MIMO. Channel bandwidth information used forNDP transmission may be set to a bandwidth value used for transmissionof the NDPA frame.

The STA1 321 transmits a feedback frame to the AP 310 (step S331). Thechannel bandwidth information used for transmission of the feedbackframe may be set to a value less than or equal to a channel bandwidthused for transmission of the NDPA frame.

The AP 310 transmits a feedback poll frame to the STA2 322 afterreceiving the feedback frame from the STA1 321 (step S341). The feedbackpoll frame is a frame for requesting the receiving STA to transmit thefeedback frame. The feedback poll frame is transmitted in a unicastmanner to an STA to which transmission of the feedback frame will berequested. Upon receiving the feedback poll frame, the STA2 322transmits the feedback frame to the AP 310 (step S332). Subsequently,the AP 310 transmits the feedback poll frame to the STA3 323 (stepS342). The STA3 323 transmits the feedback frame to the AP 310 inresponse to the feedback poll frame (step S333).

Various channel bandwidths may be used to transmit data in the WLANsystem. In order to estimate channels for the various bandwidths,channel information for the various bandwidths may be fed back. The nextgeneration WLAN system supports bandwidths of 20 MHz, 40 MHz, 80 MHz,contiguous 160 MHz, and non-contiguous 160 (80+80) MHz. Therefore, sincechannel information for each bandwidth is fed back, an amount of channelfeedback information may increase.

In the present invention, channel information based on channelestimation performed by an STA is transmitted by being included in afeedback frame transmitted from the STA to an AP. The feedback frameincludes a channel information field and a channel information controlfield. Table 2 and Table 3 below show formats of the channel informationcontrol field and the channel information field.

TABLE 2 Subfield Description Nc Index Indicate the number of columns ofa beamforming feedback matrix 0 if Nc = 1 1 if Nc = 2 . . . 7 if Nc = 8Nc Index Indicate the number of rows of a beamforming feedback matrix 0if Nr = 1 1 if Nr = 2 . . . 7 if Nr = 8 channel bandwidth Indicate abandwidth of an estimated channel 0 if 20 MHz 1 if 40 MHz 2 if 80 MHz 3if 160 MHz or 80 + 80 MHz grouping, Ng The number of carriers forgrouping 0 if Ng = 1 1 if Ng = 2 2 if Ng = 4 (3 is reserved) codebookIndicate a size of codebook entries information MU-scheme Indicatewhether beamforming feedback is for SU-MIMO or MU-MIMO sounding sequenceA sequence number from an NDPA that requests feedback

TABLE 3 Subfield Description SNR(signal to noise average SNR onsubcarriers in a recipient for ratio) of spatial stream a first spatialstream 1 . . . . . . SNR of spatial stream Nc average SNR on subcarriersin a recipient for an Nc^(th) spatial stream beamforming feedback orderof an angle of a beamforming feedback matrix matrix for a correspondingsubcarrier (subcarrier index 0) beamforming feedback order of an angleof a beamforming feedback matrix matrix for a corresponding subcarrier(subcarrier index 1) . . . . . . beamforming feedback order of an angleof a beamforming feedback matrix matrix for a corresponding subcarrier(subcarrier index Ns)

Information of the channel information field disclosed in Table 3 can beinterpreted based on information included in the channel control fielddisclosed in Table 2.

In the next generation WLAN system, the MU-MIMO transmission scheme issupported and thus more channels are present between an AP and aplurality of MU-MIMO paired STAs. Further, since a channel bandwidth ofa broader band is supported to increase a throughput, an amount ofchannel information to be fed back by the STA can be significantlyincreased. For example, an interference signal may be received in a partof time duration in a process of transmitting SNR information for 8spatial streams and transmitting feedback information corresponding to160 MHz bandwidth, and corruption may occur in a part which is intendedto be normally received in the time duration. An example thereof can befound in FIG. 4.

If all feedback frames are subjected to error detection by using thesame checksum, there is no way for a recipient of the feedback frames todetermine at which part an error occurs, which leads to a state wherethe whole feedback frame has to be discarded. In addition, a transmitterof the feedback frame transmits again the feedback frame by includinglarge-sized channel information. For this, a channel is occupied, andthus overall throughput of the WLAN system may deteriorate.

If a great amount of channel information has to be fed back as describedabove, it is proposed to segment the channel information on a bandwidthor a spatial stream basis and transmit the segmentations by consideringthem as respective feedback units. Although the channel information issegmented on the bandwidth basis in a method to be describedhereinafter, the present invention is also applicable when the channelinformation is segmented on the spatial stream basis.

In a case where a data sequence to be transmitted becomes too long, thedata sequence can be segmented and thus can be transmitted in a dataunit. However, if channel information to be fed back is segmented on anybit basis, a specific feedback unit itself is not self-decodable, andcan be decoded and utilized as understandable information only whenprevious bit information is known. To avoid such a problem, there is aneed to perform segmentation on a specific bandwidth or spatial streambasis so that each segment itself is decodable. For example, whenchannel information per bandwidth is fed back, the channel informationcan be segmented and then can be included in a plurality of data units,and the plurality of data units can be generated in an aggregated MACprotocol data unit (A-MPDU) and then the A-MPDU can be transmitted. Inthis case, each channel information is included in a MAC protocol dataunit (MPDU) which is a data unit exchanged by a MAC entity, and theA-MPDU has a structure including a plurality of MPDUs and can betransmitted or transported as one PSDU which is a service data unitmanaged in a physical layer.

FIG. 5 shows an example of a feedback frame applicable to an embodimentof the present invention.

Referring to FIG. 5, a feedback frame 500 according to the embodiment ofthe present invention has an A-MPDU format, and can be transmitted in aPPDU format by appending a preamble and a PLCP header. The feedbackframe 500 includes four sub-feedback frames 510, 520, 530, and 540.However, a specific number of subframes shown in FIG. 5 is for exemplarypurposes only, and thus the number of subframes may be greater than orequal to 4 according to a size of channel information. Each sub-feedbackframe may have an MPDU format. Each sub-feedback frame includes segmentfeedback channel information. The segment feedback channel informationmay be channel information for a specific spatial stream or channelinformation for a specific bandwidth. In the present embodiment, thesegment feedback channel information implies channel information for aspecific channel bandwidth. A plurality of pieces of segment feedbackchannel information may be configured to have the same amount except fora last segment.

Referring to FIG. 5, since an interference signal is detected togetherat a time of receiving the sub-feedback frame #4 540, the AP is unableto normally acquire 4^(th) segment feedback information. According tothe conventional channel sounding method, retransmission is requestedfor whole feedback channel information when a part of the feedbackchannel information is not correctly received due to a reception signalinterference. On the other hand, when a sounding target STA segments thefeedback channel information and transmits it in an A-MPDU formataccording to the embodiment of the present invention, even if a signalinterference acts on a part of the feedback channel information,retransmission may be requested for segment feedback channel informationincluding that part. This can be performed by a method of transmitting areport poll frame to an STA in order to retransmit specific segmentfeedback channel information.

FIG. 6 shows an example of a channel sounding method according to anembodiment of the present invention.

Referring to FIG. 6, an AP 610 transmits an NDPA frame to an STA1 621and an STA2 622 (step S610). The NDPA frame is for announcinginformation identifying an STA for transmitting a feedback frame beforeNDP transmission. The AP 610 transmits the NDPA frame by inserting STAinformation field including information on sounding target STAs, i.e.,STA1 621 and STA2 622, into the NDPA frame. A format of the STAinformation field included in the NDPA frame and included informationare as shown in the STA information field described with reference toFIG. 3.

Subsequent to transmission of the NDPA frame, the AP 610 transmits anNDP frame to the STA1 621 and the STA2 622 (step S620). The NDP issubjected to be beamformed by the AP 610, and is transmitted to the STA1621 and the STA2 622 by using at least one or more spatial streams.Therefore, the STA1 621 and the STA2 622 may estimate channels by usinga VHT-LTF of the NDP. A plurality of pieces of control informationincluded in the NDP are the same as those of the NDP described withreference to FIG. 3.

The STA1 621 transmits a feedback frame to the AP 610 (step S631).Channel bandwidth information used for transmission of the feedbackframe can be configured to have a bandwidth narrower than or equal to achannel bandwidth used for transmission of the NDPA frame. A format ofthe feedback frame includes four sub-feedback frames including segmentfeedback information similarly to the feedback frame format of FIG. 5.

The sub-feedback frame to be transmitted by being included in thefeedback frame is transmitted by including one piece of segment feedbackchannel information, and when transmitting feedback channel informationfor a wide bandwidth, it can be transmitted by using several pieces ofsegment feedback channel information. In this case, in order to make thesegment feedback information self-understandable/decodable, the feedbackinformation is transmitted by further including control informationrequired to interpret the segment feedback channel information for eachpiece of segment feedback channel information. This will be described ingreater detail with reference to FIG. 7 and FIG. 8.

FIG. 7 is a diagram showing a format of a feedback control field thatcan be included in a segment feedback frame according to an embodimentof the present invention. A segment feedback control field includescontrol information required to interpret segment feedback informationincluded in the segment feedback frame.

Referring to FIG. 7, a feedback control field 700 includes an Nc indexsubfield 710, an Nr index subfield 720, a channel bandwidth subfield730, a grouping subfield 740, a codebook information subfield 750, amulti user (MU)-type subfield 760, a segment information subfield 770,and a sounding sequence subfield 780. The subfields other than thesegment information subfield 770 include information of Table 2, and canbe configured depending on the information.

The segment information subfield 770 includes information indicating towhich segment the segment feedback information belongs. The segmentinformation subfield includes a remaining-segment subfield 771 and afirst-segment subfield 772.

The remaining-segment subfield 771 includes information indicating thenumber of pieces of remaining-segment feedback channel information in anassociated feedback frame. For example, if a currently confirmedremaining-segment subfield value is 5, five pieces of feedback channelinformation are more present. When a segmented feedback frame istransmitted or when a non-segmented feedback frame is transmitted, theremaining-segment subfield 771 is set to ‘0’ when transmitted. When thesegment feedback channel information is retransmitted, theremaining-segment subfield 771 can be set to the same value which is setfor the segment feedback channel information in original transmission.

The first-segment subfield 772 may be set to ‘1’ if specific segmentfeedback channel information of the segmented feedback frame correspondsto a first segment or if a non-segmented feedback frame is transmitted,and may be se to ‘0’ in other cases. When segment feedback channelinformation is retransmitted, the first-segment subfield 772 may be setto be equal to a value which is set for the segment feedback channelinformation in original transmission.

Upon receiving a sub-feedback frame, the AP can know a specific order ofsegment feedback channel information included in the frame by usingvalues which are set in the remaining-segment subfield 771 and thefirst-segment subfield 772, and can determine whether a specific segmentfeedback channel is normally received. For example, if a sub-feedbackframe of which the first-segment subfield 772 is set to ‘1’ is notsuccessfully received, it can be determined that reception of firstsegment feedback channel information fails. In addition, values of theremaining-segment subfield 771 can be checked for to determine apresence or absence of missing segment feedback channel informationaccording to whether the values are sequentially decreased from aspecific number to zero.

FIG. 8 is a diagram showing a channel information field format that canbe included in a segment feedback frame according to an embodiment ofthe present invention. Segment feedback channel information transmittedby an STA is transmitted by being included in a channel informationfield.

Referring to FIG. 8, a channel information field 800 includes aplurality of SNR subfields 810 indicating an average SNR value for aspatial stream and a feedback matrix subfield 820. Each SNR subfieldindicates a recipient-side SNR value for a specific spatial stream orspatial stream set. The feedback matrix subfield 820 includes a feedbackmatrix related value as segment feedback information.

Meanwhile, since one time transmission is enough for informationindicating an SNR value for a spatial stream, it may be unnecessary totransmit the information by inserting it into each of a plurality ofsegment feedback frames. Therefore, a feedback frame can be configuredsuch that a specific segment feedback frame includes an SNR subfield butother segment feedback frames do not include the SNR subfield.Alternatively, according to implementation, the SNR subframe can beincluded in all segment feedback frames when transmitted. This isbecause although the SNR subfield is included in one segment feedbackframe when transmitted, a delay may occur until SNR information for eachspatial stream is received again if an AP and/or a reception STA cannotnormally receive the segment feedback frame.

Referring back to FIG. 6, signal interference does not occur when theSTA1 621 receives sub-feedback frames 1, 2, and 4, and thus decoding isnormally performed and 1^(st), 2^(nd), and 4^(th) pieces of segmentfeedback channel information are normally acquired. On the other hand,when receiving a sub-feedback frame 3, a signal interference 60 occursand thus decoding cannot be normally performed on a 3^(rd) sub-feedbackframe, and eventually a 3^(rd) piece of segment feedback channelinformation may not be acquired. Therefore, the AP 610 has to requestthe STA1 621 to retransmit the 3^(rd) piece of segment feedback channelinformation which cannot be decoded since it fails to be receivednormally.

The AP 610 transmits a feedback report poll frame to request the STA1621 to retransmit the 3^(rd) piece of segment channel feedbackinformation (step S641). The feedback report poll frame transmitted bythe AP 610 includes information indicating segment feedback channelinformation which is desired to be retransmitted. In particular, ifindication information for requesting any segment feedback channelinformation among a plurality of pieces of segment feedback channelinformation is included, the indication information can be implementedin a bitmap format. For this, the feedback report poll frame may includea segment retransmission indication field. If a specific bit is set to‘1’ in a bit-stream constituting the segment retransmission indicationfield, it can be interpreted that retransmission of segment feedbackchannel information related to that bit is requested. However, it can beconfigured such that a value ‘0’ implies a retransmission request. Inthe example of FIG. 6, the bit-stream constituting the segmentretransmission indication field included in the feedback report pollframe transmitted by the AP 610 in step S641 can be set to ‘0 0 1 0’.

The STA1 621 may receive the feedback report poll frame and confirm avalue of the segment retransmission indication field and thus may knowthat retransmission of the 3^(rd) piece of segment feedback channelinformation is necessary. The STA1 621 transmits a feedback frame to theAP 610 in response to the feedback report poll frame (step S632). Thefeedback frame may include only the 3^(rd) piece of segment feedbackchannel information which is retransmission target segment feedbackchannel information indicated by the segment retransmission indicationfield or may include all pieces of segment feedback channel information.The retransmitted feedback frame may include a feedback control fieldand a channel information field as shown in the format of FIG. 7 andFIG. 8.

The AP 610 receives the segment feedback channel information of whichretransmission is requested from the STA1 621, and thereafter transmitsa feedback report poll frame to the STA2 622 (step S642). Since the AP610 has to request the STA2 622 to transmit the whole feedback channelinformation, the report poll frame may include a segment retransmissionindication filed which is configured to indicate the whole segmentfeedback channel information. As such, when performing a channelsounding method, if the AP 610 intends to acquire channel feedbackinformation from an STA other than a target STA to which a feedbackframe will be first transmitted, the feedback report poll frameincluding the segment retransmission indication field configured toindicate the entire segment feedback channel information can betransmitted to the STA.

The STA2 622 transmits to the AP 610 a feedback frame including channelestimation information in response to the report poll frame receivedfrom the AP 610 (step S633). The feedback frame transmitted by the STA2622 may include the feedback control field and the channel informationfield as shown in the format of FIG. 7 and FIG. 8.

In the defining of the segment feedback information, the embodiment ofthe present invention proposes to additionally divide a bandwidth into20 MHz, 40 MHz, or 80 MHz.

FIG. 9 shows a conceptual location of a subcarrier tone to be fed backaccording to an embodiment of the present invention.

Referring to FIG. 9, feedback information of 80 MHz may be divided by 20MHz to generate a plurality of pieces of segment feedback channelinformation, and a sub-feedback frame based on a MPDU format may beconfigured for each piece of segment feedback channel information andthen may be fed back in an A-MPDU format. A reference bandwidth used forsegmentation may be not only 20 MHz but also 40 MHz or a specificbandwidth higher than 40 MHz. When transmitting the feedback channelinformation by dividing the information in this manner, the segmentfeedback channel information needs to be indexed and indicationinformation thereof needs to be provided.

In addition, when an AP performs channel sounding on one STA, a reportpoll frame is not necessarily used to perform sounding, but a feedbackmatrix or feedback information such as a channel coefficient can beindividually acquired by transmitting an NDPA frame and an NDP frame.

In a case where specific segment feedback channel information is notnormally received when receiving a feedback by attempting sounding onetime, a method capable of selectively receiving the specific segmentfeedback channel information again by using the NDPA frame and the NDPframe is proposed.

For this, information indicating that the specific segment feedbackchannel information will be selectively transmitted may be included inthe NDPA frame when transmitted. As such, when retransmission of thesegment feedback channel information is requested by reusing the NDPAframe and the NDP frame, since this is a case where retransmission isrequested only for specific one STA, the STA information field of theNDPA frame may include only a field for one STA.

In addition, when the AP performs channel sounding on a plurality ofSTAs, it is possible to request a specific STA to feed back onlyspecific feedback channel information.

FIG. 10 shows an example of an NDPA frame format according to anembodiment of the present invention.

Referring to FIG. 10, an NDPA frame 1000 includes a frame control field1010, a duration field 1020, a receiver address (RA) field 1030, atransmitter address (TA) field 1040, a sounding sequence field 1050, atleast one or more STA information fields 1060, and an FCS field 1070.

The frame control field 1010 includes control information for the NDPAframe 1000. The duration field 1020 includes length information of theNDPA frame 1000. The RA field 1030 can be set to a broadcast address,but if sounding is requested to one specific STA, can be set to a MACaddress of the STA. The TA field 1040 can be set to a MAC address of anAP that transmits the NDPA frame 1000. The sounding sequence field 1050is set to a sequence number for a current sounding sequence. The FCSfield 1070 includes CRC related information for frame transmission andreception.

An STA information field #1 1061 and an STA information field #2 1062include AID subfields 1061 a and 1062 a indicating AIDs of soundingtarget STA1 and STA2, feedback type subfields 1061 b and 1062 bindicating a feedback request type for the sounding target STA, and Ncindex fields 1061 c and 1062 c indicating a feedback dimension. Thenumber of STA information fields 1060 can change depending on the numberof sounding target STAs, and sounding requested to two STAs in thepresent embodiment is for exemplary purposes only.

However, when the AP requests the STA1 to selectively retransmit segmentfeedback channel information, the STA information field 1060 for theSTA1 may further include a segment retransmission indication subfield1061 d. The segment retransmission indication subfield 1061 d can beconfigured similarly to the segment retransmission indication subfieldincluded in the aforementioned report poll frame.

When the AP broadcasts an NDPA frame having the aforementioned frameformat, the STA1 and the STA2 receive the NDPA frame 1000 and can knowthat it is an NDPA frame for the STA1 and the STA2 by using the AIDsubfields 1061 a and 1062 a of the STA information fields 1061 and 1062.Thereafter, when the AP sends the NDP frame, each feedback frame istransmitted to the AP, and in this case, the STA1 may feed back onlysegment feedback channel information indicated by the segmentretransmission indication subfield 1061 d included in the NDPA frame1000.

FIG. 11 is a block diagram showing a wireless apparatus to which anembodiment of the present invention is applicable. The wirelessapparatus may be an AP or an STA.

The wireless apparatus 1100 includes a processor 1110, memory 1120, anda transceiver 1130. The transceiver 1130 transmits and/or receives aradio signal and implements the physical layer of the IEEE 802.11standard. The processor 1110 is operatively coupled to the transceiver1130 and implements the MAC layer and the PHY layer of the IEEE 802.11standard.

A processor 1110 can be configured to generate and transmit an NDPAframe, an NDP, and a feedback report poll frame proposed in the presentinvention, and can also be configured to receive the transmitted frameand interpret an included field value in order to acquire controlinformation. In addition, the processor 1110 can be configured totransmit feedback channel information in association with requestinformation included in the frame or can be configured to selectivelytransmit segment feedback channel information. The processor can beconfigured to implement the aforementioned embodiment of the presentinvention described with reference to FIG. 2 and FIG. 10.

The processor 1110 and/or the transceiver 1130 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 1120 and may be performed by the processor 1110. The memory 1120may be located inside or outside the processor 1110, and may be coupledto the processor 1110 by using various well-known means.

According to a channel sounding procedure of the present invention, achannel bandwidth of a wideband and channel information increased due tothe support of a multi user-multiple input multiple output (MU-MIMO)transmission scheme are separately transmitted. In doing so, even if apart of full channel information to be fed back is lost, the remainingparts can be utilized as normal channel information, thereby increasingreliability of frame transmission.

In addition, when the part of the full channel information to be fedback is lost, retransmission can be requested for the channelinformation and thus retransmission can be performed in responsethereto. In doing so, a time required for unnecessarily accessing to achannel for the channel sounding procedure can be reduced, therebyincreasing efficiency of a channel sounding method and improving athroughput of an overall wireless location area network (WLAN).

Although the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A method for channel sounding in a wireless local area network, the method performed by a transceiver and comprising: receiving a null data packet announcement (NDPA) frame from a transmitter to initiate the channel sounding; receiving a null data packet (NDP) from the transmitter, the NDP following the NDPA frame; initiating the channel sounding with the transmitter, the channel sounding including transmitting a plurality of segments; receiving a retransmission request from the transmitter, the retransmission request including a segment retransmission bitmap field indicating at least one segment of the plurality of segments for retransmission; and retransmitting the at least one segment indicated in the segment retransmission bitmap field.
 2. The method of claim 1, wherein a bit of the segment retransmission bitmap field is set to 1 to indicate a corresponding segment is requested, and is set to 0 to indicate the corresponding segment is not requested.
 3. The method of claim 1, wherein the step of transmitting the plurality of segments comprises: generating a frame including a first report field and a second report field, wherein the first report field includes beamforming information and the second report field includes SNR information, and wherein the first and second report fields are split into the plurality of segments; and transmitting the plurality of segments in different medium access control (MAC) protocol data units (MPDUs).
 4. The method of claim 1, further comprising: splitting the first and second report fields into the plurality of segments when a size of a MPDU corresponding to a frame that could be created when the first and second report fields are not split exceeds a maximum MPDU length.
 5. The method of claim 1, wherein the step of transmitting the plurality of segments in different medium access control (MAC) protocol data units (MPDUs) comprises: transmitting the plurality of segments in a single aggregated MPDU (A-MPDU).
 6. A device configured to perform channel sounding in a wireless local area network, comprising, a transceiver; and a controller operatively connected to the transceiver and configured to receive a null data packet announcement (NDPA) frame from a transmitter to initiate the channel sounding, receive a null data packet (NDP) from the transmitter, the NDP following the NDPA frame, initiate the channel sounding with the transmitter, the channel sounding including transmitting a plurality of segments, receive a retransmission request from the transmitter, the retransmission request including a segment retransmission bitmap field indicating at least one segment of the plurality of segments for retransmission, and retransmit the at least one segment indicated in the segment retransmission bitmap field.
 7. The device of claim 6, wherein a bit of the segment retransmission bitmap field is set to 1 to indicate a corresponding segment is requested, and is set to 0 to indicate the corresponding segment is not requested.
 8. The device of claim 6, wherein the controller is configured to: generate a frame including a first report field and a second report field, wherein the first report field includes beamforming information and the second report field includes SNR information, and wherein the first and second report fields are split into the plurality of segments, and transmit the plurality of segments in different medium access control (MAC) protocol data units (MPDUs).
 9. The device of claim 6, wherein the controller is configured to split the first and second report fields into the plurality of segments when a size of a MPDU corresponding to a frame that could be created when the first and second report fields are not split exceeds a maximum MPDU length.
 10. The device of claim 6, wherein the controller is configured to transmit the plurality of segments in a single aggregated MPDU (A-MPDU).
 11. A method for channel sounding in a wireless local area network, the method performed by a transceiver and comprising: transmitting a null data packet announcement (NDPA) frame to a receiver to initiate the channel sounding; transmitting a null data packet (NDP) to the receiver, the NDP following the NDPA frame; initiating the channel sounding with the receiver, the channel sounding including receiving a plurality of segments; transmitting a retransmission request from the transmitter, the retransmission request including a segment retransmission bitmap field indicating at least one segment of the plurality of segments for retransmission; and receiving the at least one segment indicated in the segment retransmission bitmap field.
 12. The method of claim 11, wherein a bit of the segment retransmission bitmap field is set to 1 to indicate a corresponding segment is requested, and is set to 0 to indicate the corresponding segment is not requested.
 13. The method of claim 11, wherein the plurality of segments comprises: a frame including a first report field and a second report field, wherein the first report field includes beamforming information and the second report field includes SNR information, wherein the first and second report fields are split into the plurality of segments, and wherein the plurality of segments are received in different medium access control (MAC) protocol data units (MPDUs).
 14. The method of claim 11, wherein the first and second report fields are split into the plurality of segments when a size of a MPDU corresponding to a frame that could be created when the first and second report fields are not split exceeds a maximum MPDU length.
 15. The method of claim 11, wherein the plurality of segments are received in a single aggregated MPDU (A-MPDU).
 16. A device configured to perform channel sounding in a wireless local area network, comprising, a transceiver; and a controller operatively connected to the transceiver and configured to transmit a null data packet announcement (NDPA) frame to a receiver to initiate the channel sounding, transmit a null data packet (NDP) to the receiver, the NDP following the NDPA frame, initiate the channel sounding with the transmitter, the channel sounding including receiving a plurality of segments, transmit a retransmission request from the transmitter, the retransmission request including a segment retransmission bitmap field indicating at least one segment of the plurality of segments for retransmission, and receive the at least one segment indicated in the segment retransmission bitmap field.
 17. The device of claim 16, wherein a bit of the segment retransmission bitmap field is set to 1 to indicate a corresponding segment is requested, and is set to 0 to indicate the corresponding segment is not requested.
 18. The device of claim 16, wherein the plurality of segments comprises a frame including a first report field and a second report field, wherein the first report field includes beamforming information and the second report field includes SNR information, wherein the first and second report fields are split into the plurality of segments, and wherein the plurality of segments are received in different medium access control (MAC) protocol data units (MPDUs).
 19. The device of claim 16, wherein the first and second report fields are split into the plurality of segments when a size of a MPDU corresponding to a frame that could be created when the first and second report fields are not split exceeds a maximum MPDU length.
 20. The device of claim 16, wherein the plurality of segments are received in a single aggregated MPDU (A-MPDU). 